Modern paper making processes and machinery follow the Fourdrinier method wherein an aqueous dispersion of paper making fibers is poured onto a high speed traveling woven fabric through which water from the dispersion drains leaving a thin web of wet fibers which is dried and finished to a sheet of paper. The key step in this method is that of forming the web from the fiber/aqueous dispersion. This must be done very quickly and uniformly across the width of the endless fabric. Normally, the transition of dewatering commences by gravity, followed by other means such as foil blades, continuing with a plurality of controlled low vacuum boxes and then by a plurality of high vacuum boxes. There are many causes for mishaps to occur that prevent the final sheet of paper from being perfect. One of the principal causes is that air may penetrate the web off paper and the fabric causing nonuniformities in the paper. Such disturbances may be caused by nonuniform drainage at every square inch of the fabric surface, and entrainment of air in the fiber/aqueous dispersion, followed by forcing air through the dispersion and fabric whereby air will find the path of least resistance and fixing the flocculation of such dispersion unevenly over the fabric. The demand for higher and higher speed makes it increasingly difficult to produce a paper sheet that is isotropic.
The critical step of this process is the water removal, which must be done quickly and uniformly in order to obtain a layer of fibers on the fabric that can be finished to a high quality paper. The principal difficulty in producing a fast, uniform drainage has been that when the drainage is speeded up by applying a vacuum there are numerous instances at random locations across the fabric where air will be pulled through the layer of wet fibers. At each location a small vortex appears to break the continuity of the film of water and fiber on the fabric, and to permit the passage of air through the entire film and thereby disrupting the uniform settling of the fibers into a web of uniform thickness and strength. Every time such an instance occurs, a meniscus is formed at the interface of the water and air and this is an obstruction to the free uniform flow of water away from the fibers forming the web. The formation of such air holes through the mass of fibers forming the web must be minimized if any improvement in sheet formation at high speed is to be achieved.
An improved method and apparatus for sheet formation in the Fourdrinier paper is disclosed in our copending U.S. patent application Ser. No. 07/717,880 filed Jun. 17, 1991, now U.S. Pat. No. 5,242,547. That patent describes an improved procedure for maintaining a continuous drainage of water with substantially no air flow discontinuities occurring in the forming web. The apparatus described in that application relates primarily to the wetter end of the web immediately following the head box.
It is an object of this invention to provide an apparatus especially suited for use downstream of the head box and the wetter drainage box or boxes and upstream of the couch roll. One of the advantages of this invention is to decrease the amount of friction between the fabric and the dewatering components and to give the fabric a longer useful life. Another advantage is to substantially reduce the length of the forming area of the fabric, and thereby to reduce the number of dewatering components required.
Other advantages include:
A. retention of more chemical additives and fines due to the more gentle dewatering and uniformity of dewatering; PA1 B. easier release of web from the fabric due to the web not being forced into the interstices of the fabric by high vacuum whereby a web pick-up vacuum roll or high pressure air from below the fabric not needed; PA1 C. amount of defoamer is reduced; PA1 D. enhanced sheet strength; and PA1 E. enhanced drying at the end of the fabric (couch roll) thus reducing the power used in the pressing and/or the drying sections. Still other objects will appear from the more detailed description which follows. PA1 A. Meniscus is the surface of a water volume which is in contact with a dissimilar surface. The dissimilar surface may be the container holding the water or the gases in contact with a surface of the water or surrounding the water, such as air when a drop of water is falling through it. Webster's New International Dictionary, 2nd Edition, Unabridged, 1934 defines Meniscus as --the curved upper surface of a liquid column that is concave when the containing walls are wetted by the liquid and convex when not. The meniscus is present at the interface between the liquid and the vessel in which it is contained. PA1 B. Surface Tension is a condition that exists at the free surface film of a liquid by reason of intermolecular forces about the individual surface molecules and is manifested by properties resembling those of an elastic skin under tension. Surface Tension is a characteristic of the waiter meniscus which can be modified by chemical means. The meniscus changes its geometric (concave shape depending on the Surface Tension of the liquid.